Our invention relates to indicating and display apparatus and more particularly to indicator circuits using luminous elements in switching and control systems.
In switching, control and other electronic systems, visual indicators are generally utilized for display and control purposes. Each visual indicator displays the status of a particular system function or the state of a device and may comprise a plurality of luminous elements and associated electronic circuitry. Typical control systems may employ several dozen lamps to display the system status, and typical switching systems, such as telephone offices, may include several hundred lamps to provide visual indication of the status of equipment used to route calls through the office.
Generally, each indicator circuit comprises a luminous element, such as a light-emitting diode (LED), placed in series with a resistor and a switching device. The switching device may comprise transistors or other semiconductor elements. A plurality of such circuits are paralleled and connected to the available voltage source to assure proper lumination for the expected variations in element characteristics. Such arrangements, however, are relatively inefficient, owing to the power loss in each series dropping resistor. Even a moderate power loss in an individual circuit results in a relatively high power loss where several hundred circuits are used.
One way to reduce the power requirements of the indicator circuits is to transform the high voltage available into a lower voltage, thereby reducing the series dropping resistor loss. This method, however, requires the use of additional equipment, high current capacity devices to accomplish the voltage transformation, and involves a power loss in the transformation equipment. Additionally, there is a minimum voltage usable in the indicator circuits because of the variations in the electrical characteristics of the luminous elements, and there is a minimum current which must be supplied to the luminous elements to assure sufficient illumination for visual display purposes. It is an object of the invention to provide an economical indicator circuit for a plurality of independently operative luminous elements with improved efficiency.